Summary We demonstrate here that LXR–dependent sterol homeostasis is a physiologically-regulated determinant of cell proliferation and acquired immune responses. T cell activation triggers simultaneous suppression of the LXR pathway for cholesterol transport and induction of the SREBP pathway for cholesterol synthesis. This coordinated program is engaged in part through induction of the sterol-metabolizing enzyme SULT2B1, expression of which in T cells blocks LXR signaling. Forced induction of LXR target genes during T cell activation markedly inhibits mitogen-driven expansion, whereas loss of LXRβ confers a proliferative advantage. Inactivation of the sterol transporter ABCG1 in T cells uncouples LXR signaling from proliferation, directly linking sterol homeostasis to the anti-proliferative action of LXR. Mice lacking LXRβ exhibit lymphoid hyperplasia and enhanced responses to antigenic challenge, indicating that proper regulation of LXR-dependent sterol metabolism is important for immune responses. These data implicate LXR signaling in a metabolic checkpoint that modulates cell proliferation and immunity.

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Summary Effective clearance of apoptotic cells by macrophages is essential for immune homeostasis. The transcriptional pathways that allow macrophages to sense and respond to apoptotic cells are poorly defined. We demonstrate here that LXR signaling is important for both apoptotic cell clearance and the maintenance of immune tolerance. Apoptotic cell engulfment activates LXR and thereby induces the expression of Mer, a receptor tyrosine kinase critical for phagocytosis. LXR null macrophages exhibit a selective defect in phagocytosis of apoptotic cells and an aberrant pro-inflammatory response to them. As a consequence of these defects, mice lacking LXRs manifest a breakdown in self-tolerance and develop autoantibodies and autoimmune glomerulonephritis. Treatment with an LXR agonist ameliorates disease progression in mouse models of Lupus-like autoimmunity. Thus, activation of LXR by apoptotic cells engages a virtuous cycle that promotes their own clearance and couples engulfment to the suppression of inflammatory pathways.